We consider an Ising model confined in an Lx infinity geometry with identical surface fields at the boundaries. According to the Kelvin equation the bulk coexistence field scales as 1/L for large L; thermodynamics and scaling arguments predict higher-order corrections of the type 1/L-2 and 1/L-5/3 at partial and complete wetting, respectively. Our numerical results, obtained by density-matrix renormalization techniques for systems of widths up to L=144, are in agreement with a 1/L2 correction in the partially wet regime. However, at complete wetting we find a large range of surface fields and temperatures with a correction to scaling of type 1/L-4/3. We show that this term is generated by a thin wetting layer whose free energy is dominated by the contacts with the wall. For L sufficiently large we expect a crossover to a 1/L5/3 correction as predicted by the theory for a thick wetting layer. This crossover is indeed found in a solid-on-solid model, which provides a simplified description of the wetting layer and allows the study of much larger systems than those available in a density-matrix renormalization calculation.